[e] (0) Add a note mentioning how forced in-band subtitles should trigger
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=23507
Affected topics: Video Text Tracks

git-svn-id: http://svn.whatwg.org/webapps@8500 340c8d12-0b0e-0410-8428-c7bf67bfef74